[TASK][MEDIUM] Engine adaptive pool size

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### Skill requirements

- Basic knowledge of Scala programming language
- Familiar with the architect of Apache Kyuubi

### Background and Goals

In `kyuubi.engine.share.level` non-CONNECTION mode, how many engines the Kyuubi server starts is determined by `kyuubi.engine.pool.size`.

The engine selection policy is determined by `kyuubi.engine.pool.selectPolicy`. The default is `RANDOM`, which means randomly selecting a number within the engine pool size range and checking whether the engine corresponding to this number has been created. If not, create one.

When `kyuubi.engine.pool.size` is a relatively large value, it may randomly create many engines.

In addition, `kyuubi.session.engine.idle.timeout` configures a relatively large idle time, so that once the engine is created, it will be slower to release the engine.

This selection strategy is a bit of a waste of resources, especially since it takes a while to start the spark cluster mode.

### Implementation steps

We can expose some information about the engine load in the Engine. When the server selects the engine, it will decide whether to use a new engine or reuse the old engine based on the load.

### Additional context

Original reporter is @cxzl25

Introduction of https://github.com/apache/kyuubi/issues/6232